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What is the use of "extern C" in C++ code?

+7 votes
448 views

In lot of C++ code in my company I see extern C code something like 

extern "C"
{
    int sum(int x, int y)
    {
        return x+y;
    }
}

Please explain the significance of this?

posted Apr 27, 2016 by anonymous

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1 Answer

0 votes

When you write a C++ program and compile the program using the g++ compiler, it does name mangling which is related to somewhat function overloading concept. By putting the code within the extern "C" { } block, does not allow g++ compiler to do name mangling for functions written in C .

answer Apr 27, 2016 by Harshita
Harshita,
i created 2 function with same name and different number of arguments,
1 function i put it into extern "C" and other outside of it,
While compiling it i am able to compile and run it.
Can you please tell me where i am making mistake to understand extern "C"?
commented Apr 28, 2016 by Chirag Gangdev
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+1 vote
Warning for incompatible functions declared 'extern "C"'?

I have an oversight in my code where I'm declaring & defining a function
with C-linkage, though it's not possible.

Example snippet:

#ifdef __cplusplus
extern "C"
{
#endif

// ... some functions which are compatible with C linkage

// Intended to be a helper function not exposed from library
std::string GetEngineVersion()
{
 // ...
}

#ifdef __cplusplus
}
#endif

Obviously the GetEngineVersion function cannot have C linkage because it returns a C++ class.

My question is: does GCC have a warning for this scenario? Specifically, can it warn when something is declared extern "C" that's incompatible with C linkage?

I compile with the following warning flags and I didn't get a warning:

-Wall -Wunused-parameter -Wextra -Weffc++ -Wctor-dtor-privacy
-Wnon-virtual-dtor -Wreorder -Wold-style-cast -Woverloaded-virtual
-Werror

Incidentally, when I compile the same code in VS2013 I get this warning:

warning C4190: 'GetEngineVersion' has C-linkage specified, but returns UDT 'std::basic_string' which is incompatible with C
...